1. Field of the Invention
This invention relates in general to the field of semiconductor devices, and in particular, to a voltage variable resistor formulated from HBT epitaxial layers.
2. Description of the Prior Art
Power amplifier devices are typically designed to achieve a fairly linear response of gain or power versus regulated supply voltage (Vreg) over their operating design range. This is an especially desirable feature in battery operated radio communication devices such as cellular telephones which have to operate in an environment in which the supply (battery) voltage is constantly changing. Using voltage variable resistors in the design as is known in the art can effect automatic adjustment of the power amplifier gain and or power characteristics with respect to changes in Vreg. The control voltage used to adjust the resistance of the voltage variable resistor in a typical radio application could be a voltage derived from an automatic gain control circuit or some other circuit as is known in the art.
It would be beneficial if the voltage variable resistor could be integrated into the power amplifier""s semiconductor design and take up minimum surface area. Such integration is especially desirable if the voltage variable resistor is to be manufactured on heterojunction bipolar transistor (HBT) epitaxial material as are some power amplifier integrated circuits. As such, a need exists in the art for a voltage variable resistor that is manufactured on HBT epitaxial material and that is manufactured in conjunction with an HBT on the same III-V semiconductor chip.
Layered epitaxial structures providing HBTs and FETs on the same semiconductor substrate are known. U.S. Pat. No. 5,250,826, issued Oct. 5, 1993, to Chang et al., entitled PLANAR HBT-FET DEVICE, discloses a III-V compound planar HBT-FET device integrating FETs with HBTs formed on the same semiconductor substrate. Although it is known that FET structures are configurable to provide a transferred or controlled resistance under certain operating conditions, the ""826 patent does not disclose using an FET or other like device as a voltage controlled resistor, but instead focuses on using an FET as a traditional transistor for amplifying and switching applications. Further, the ""826 patent discloses a FET structure having a Schottky gate metal contact deposited on the emitter cap layer for providing a control voltage to the FET.
The present invention is directed to a voltage controlled resistance device such as a voltage controlled resistor, manufactured on HBT epitaxial material in conjunction with an HBT on the same III-V semiconductor chip. Preferably, the device can exert control over the range of 0-3.0 volts, which is the supply voltage range present in most modern battery operated devices. The device can be made with no modification to the epitaxial layers required to make AlGaAs/GaAs HBT devices; and it can be made without the necessity of a Schottky gate electrode such as required for the HBT-FET device disclosed and claimed in U.S. Pat. No. 5,250,826, discussed above. One embodiment of the invention comprises a heavily doped base layer of P-type GaAs; a lightly doped emitter layer of N-type AlGaAs deposited atop a portion of the base layer; first and second emitter mesas formed from a heavily doped emitter contact layer of N-type GaAs deposited atop the emitter layer; a heavily doped emitter cap layer of N-type InGaAs deposited atop the emitter contact layer, wherein the first and second emitter mesas are separated by a recess; emitter metal contacts deposited atop the emitter cap layer atop the first and second emitter mesas; and a base metal contact deposited atop the base layer, wherein the base metal contact is separated from the emitter layer. The recess is disposed through the InGaAs cap layer and GaAs contact layer via etchant formulations that are chemically selective to the AlGaAs emitter layer. Such selective formulations are very well known in the industry.
Accordingly, a feature of the present invention includes provision of a voltage variable resistor formulated from HBT expitaxial layers with no modification to the epitaxial structure generally associated with NPN HBT devices.
Another feature of the present invention includes provision of a voltage variable resistor via a chemical etchant that increases the manufacturing margin associated with like HBT devices.
Still another feature of the present invention includes provision of a voltage variable resistor that controls a resistance level via an electrode voltage very much like a field effect transistor, but without use of a Schottky gate electrode.
Yet another feature of the present invention includes provision of a voltage variable resistor that can be formulated with no more than two additional manufacturing operations beyond those necessary to fabricate an HBT amplifier.
Yet another feature of the present invention includes provision of a voltage variable resistor formulated from HBT epitaxial layers and having a control voltage range between 0 and about 3.0 volts dc.